THE WALL---Why?

It's a matter of electrons. Specifically, how many electrons can get to your amp when there is a sudden draw. (The wave description is equally valid.) Engineers call this source impedance - ideally, you want a source with zero impedance, like a dedicated power station.

If a dedicated power station is out of your budget, try a large isolation transformer which will clean up the power but not increase the source impedance. Plitron makes a good one - toroidal - good enough for medical equipment - and they sell to the public. Works for me, for all my equipment. YMMD

Should have said that toroids tend to growl when they are doing their job, so it's better to site them in a garage or utility room.

I have my  entire tubed based system including my amp, into my power plant.
The PP just seems to add a  subtle dimension of clarity or clearness to the sound.
The amp straight into the wall still sounds nice, but I prefer the PP S

I have my plasma on a rollaway. On movie nights, I position it between the speakers and plug it into the PP as well. Beautiful picture along with amazing audio.

I wouldn't listen/watch without one.

There are many types of PS Audio Power Plants so I'm not sure which you are speaking of. Personally if I had one of these http://www.psaudio.com/products/p3-power-plant/ I'd certainly plug my amp and everything else into it. I don't by the amp into wall outlet theory. In my case I've taken a far less expensive route: Dedicated (separate breaker) 20 AMP wall outlet >>> Emotiva CMX-2 >>> Panamax MR-4300 >>> all audio video equipment. Works well for me. Doesn't limit current by any practical standards, although technically this is not 100% passive but is extremely close to 100%. Also, I'm using SS equipment and I'm uncertain if this would work as well in a tube situation.

I've always had the best performance when amps are plugged directly into the wall.  I am using dedicated 20A circuits, but anytime I put something between the outlet and my amp, I lost something.